ELECTROMAGNETS. MAGNETISM OF IRON. 



T~* ^ i ^^ \ 7~~* ^^ 



/ y = I- I s or r = 



STT \ s ) 8-TT.y 



and the total pull due to both field cores is 



<J>2 



F= - (11) 



47TS 



When the desired pull F is given in dynes the requisite flux <3> 

 may be determined from this equation, and the magnetomotive 

 force required to produce this flux may then be calculated as 

 explained in Art. 14 or in Art. 15. 



(&) Calculation of the amount of field excitation required for a 

 dynamo. From the prescribed electromotive force, speed, and 

 number of armature conductors Z, the necessary amount of mag- 

 netic flux 4> through the armature may be calculated from equa- 

 tion (21^) of Chapter II. The various parts of the magnetic cir- 

 cuit of a dynamo, namely, the yoke, the field cores, the pole 

 pieces, the air gaps, and the armature core, do not as a rule have 

 perfectly definite lengths in the direction of the flux, nor perfectly 

 definite sectional areas at right angles to the flux. Therefore, it 

 is usually desirable to make an outline sketch of the magnetic 

 circuit of the dynamo to scale, as in the above example (a), and 

 estimate the mean length /, and the mean sectional area of each 

 part of the magnetic circuit. This done, and the magnetic prop- 

 erties of the iron of each part being given by suitable cB and df 

 curves, the required number of ampere-turns may be found as 

 explained in Art. 14 or in Art. 15. Dividing the required field 

 excitation expressed in ampere-turns by the number of turns of 

 wire in the field windings, gives the required field current in 

 amperes. 



17. Magnetic leakage. It has been already pointed out that 

 in most cases magnetic flux is forced through a magnetic circuit 

 by a bunched winding, so that the magnetomotive force is largely 

 concentrated in one part A of the circuit. Therefore the mag- 

 netic flux in passing through a portion B of the circuit, which is 



